Drawing lines in 3D space using mouse

Hello everyone!
I'm having a bit of trouble figuring out how to draw a line in 3D space.
What I want to do is allow the user to use their mouse to draw a line anywhere in the 3D space.
They would left-click, hold it, and drag the mouse, as they drag the mouse the line would follow the cursor
to where ever the new mouse position will be located.
when they release the mouse button, the line should be created.
here is a snippit of my codes:

I did use ortho-projection, but i didn't have any luck either. I've been looking around online and most suggest use Matrix projection and Ortho-projection to allow the user to draw the lines. But when I used Matrix Projection, i just get an empty blue screen(my program uses a skybox texture) so i want to keep the skybox texture there as well.

here is an attached image:
I have a skybox, and I also have a grid that was all rendered. The thick white line you see can only be drawn at that certain angle and if I change my camera anywhere else, I can't draw. The lines can ONLY be drawn horizontally as well, I can't draw vertically at all.https://www.dropbox.com/s/wrhr7frk2p...ScreenShot.png